1. Technical Field
The present invention relates to passenger vehicle collision mitigation systems, and more specifically to a method for determining the time to collision between a host vehicle and an oncoming target vehicle, and for determining the necessary host vehicle deceleration for bringing the host vehicle to a standstill at the moment of collision.
2. Background Art
As technology evolves and different sensors become more and more affordable, it is natural that traffic safety should profit considerably of this development. One type of safety system includes those oriented towards collision avoidance and/or mitigation by braking. Such systems generally comprise one or more sensors for detecting the external environment, usually being connected to a brake control management unit.
In the following, a host vehicle is defined as a vehicle for which a collision avoidance/mitigation system is active, and a target vehicle is a vehicle which the host vehicle is approaching and for which the host vehicle must brake in order to avoid or mitigate a collision.
Currently, most such systems are designed to avoid or mitigate collisions with receding vehicles, i.e. vehicles that are travelling over the road in the same direction as the host vehicle. A forward collision warning system is a known system that issues a warning for both receding and oncoming vehicles. However, this warning is generally issued at high speeds where the most effective single measure for collision avoidance is steering around the target vehicle. There is a conceptual difference between the ability of a vehicle to avoid collision by steering and by braking.
At relatively low velocities it is usually better to brake, and at relatively higher velocities it is generally better to avoid collision by steering. There is a certain velocity at which the two methods are equal, i.e. the velocity at which braking and steering are equally efficient in avoiding a collision, and that velocity is:
                    v        =                  2          ⁢          a          ⁢                                                    p                y                                            a                y                                                                        (        1        )            where:
v is the vehicle longitudinal speed;
py is the width of the object to avoid (considered equal to the width of the host vehicle);
a is the longitudinal acceleration achievable by the host vehicle through braking; and
ay is the maximum lateral acceleration achievable by the host vehicle.
It can be concluded that above this velocity, in order to avoid collision with an obstacle, it is more efficient to steer away from it, while below this velocity threshold it is more efficient to apply the brakes of the host vehicle.
The following discussion addresses the situations where it is more efficient to brake. The situations will be different depending on if the target vehicle is a receding object or an oncoming object. If the target is receding from the host vehicle, then the objective is that both host and target vehicles have the same velocity at the moment of collision. For oncoming target vehicles, the best result for the host vehicle is to reach a standstill at the moment of collision.
In the case of an oncoming target vehicle, to compute the time of impact is rather complex. It is desired to achieve a simple yet exact method to compute the time to collision and the needed host acceleration to avoid or mitigate collision.